My aims are to study molecular and genetic mechanisms of cellular repair of nuclear and mitochondrial DNA and the effect of cellular repair mechanisms on the induction of chromosomal and non-chromosomal gene mutations. These problems will be studied in a simple eucaryotic organism, the yeast Saccharomyces cerevisiae, where both a well-defined mutational system and mutants with repair deficiencies exist. For the study of nuclear gene mutations, we will measure reversion of various mutants of iso-1-cytochrome c. The main advantage of this system is that the nature of the lesions in the DNA of the mutants can be deduced from the alterations of the primary structure of iso-1-cytochrome c. Various initiator mutants, whose base alterations are known, will also be used to check if the specificity of mutagens is the same in eucaryotes as in procaryotes. Mutations of mitochondrial genes will be measured by the induction of cytoplasmic petites. Reversion frequencies obtained in both repair proficient and repair deficient strains will show what influence repair capacity has on the induction of mutations by physical agents such as ionizing and non-ionizing radiation and by chemical mutagens and carcinogens. Those radiation sensitive mutants which have a significant effect on mutation induction will be characterized for their ability to repair single strand breaks in DNA. We would also like to know how mono-functional alkylation-induced damage is repaired in yeast. Does yeast, a relatively simple eucaryotic microorganism, respond like other eucaryotes by excising alkylation products from DNA, or does it behave like procaryotes whose repair systems may have less recognition for alkylated DNA? Does the same repair process act on both nuclear and mitochondrial DNA? Or might we expect nuclear DNA to be repaired according to the eucaryotic mode and mitochondrial DNA to follow the procaryotic mode, in view of the bacteria-like nature of mitochondria? We will answer this question by measuring excision of alkylation products from nuclear and mitochondrial DNA. Lastly, we would like to know whether the same or different genes control repair of both types of DNA.